Merge branch 'mshReader' into 'master'

MeshIO (input, output, convert) for (volumetric) meshes in  .msh and .veg file format

![liverScreenShot2](/uploads/f396f4cc1570700aa9c90e835dd932d2/liverScreenShot2.png)New addition(s):
1. Added `MSHMeshIO` to read a mesh in .msh file format, and instantiate `imstk::VolumetricMesh` 
2. Added a method in `VegaMeshIO` to convert `imstk::VolumetricMesh` to `vega::VolumetricMesh`
3. Added a method in `VegaMeshIO` to write `imstk::Mesh` in .veg file format.
4. Added an example `testMshandVegaIO` in `Sandbox` to test these implementations (Attached is the output with this merge request).

See merge request !112